Periodically focused severed traveling-wave tube



M y 1961 D. J. BATES ET AL 2,985,791

PERIODICALLY FOCUSED SEVERED TRAVELINGWAVE TUBE Filed Oct. 2. 1958 4 Sheets-Sheet 1 04%0 7647.41 04/147 7. Fuzz,

May 23, 1961 D. J. BATES ET AL 2,935,791

PERIODICALLY FOCUSED SEVERED TRAVELING-WAVE TUBE Filed Oct. 2. 1958 4 Sheets-Sheet 2 y 23, 1961 D. .1. BATES ET AL 2,985,791

PERIODICALLY FOCUSED SEVERED TRAVELING-WAVE TUBE Filed Oct. 2, 1958 4 Sheets-Sheet 3 ELECTRON STREAM POLE msca DISC.

.ZI6I5.

44417411: 04140 7.5472 01/14:? 71 paez,

A as/W May 23, 1961 Filed Oct. 2. 1958 lira- D. J. BATES ET AL 2,985,791-

PERIODICALLY FOCUSED SEVERED TRAVELING-WAVE TUBE 4 Sheets-Sheet 4 POLE PIECE \SOLATOR ISOLATOR POLE PIECE ISOLATOR SPACER I20 ATTENUATORS IOO United States atent PERIODICALLY FOCUSE-l) SEVERED TRAVELING-WAVE TUBE David J. Bates, Rolling Hills, and Oliver T. Purl, Menlo Park, Calif., assiguors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Oct. 2, 1958, Ser. No. 764,883

7 Claims. (Cl. 3153.5)

This invention relates generally to microwave devices and particularly to a high gain, non oscillating, high power periodically focused traveling-wave tube amplifier.

In traveling-wave tubes an electron stream interacting with a propagation e.-m. wave is utilized to increase the level, or amplify, the electromagnetic energy. In order to achieve effective interaction between the stream of charged particles and the electromagnetic wave energy, the phase velocity of the e.-m. wave is decreased by means of any one of a number of different types of slowwave structures. The classical example of such structures is a helix wound about the path of the electron stream. Microwaves traversing the length of the helix do so at substantially the speed of light. However, their axial velocity is decreased by substantially the ratio of the axial helix length to the circumferential length of the wound conductor.

Another type of slow-wave structure particularly use ful at higher power and higher frequencies is the folded waveguide or interconnected cell type of slow-wave structure. In this type of structure a waveguide is effectively wound back and forth across the path of the electron stream. This provides, as with the helix, a path of propagation which is considerably longer than the axial length of the structure and hence the traveling wave may be made eifectively to propagate at nearly the velocity of the electron stream. The interactions of the electrons of this stream and the traveling wave causes velocity modulations and bunching of the beam. The net result may then be a transfer of electromagnetic energy from the electron beam to the wave travelling on the slowwave structure.

The present invention is primarily but not necessarily concerned with traveling-wave tubes utilizing slow-wave structures of the type last above mentioned, viz., the folded waveguide or interconnected cell type. Modern practical techniques for fabricating this type of slow-wave structure usually provide a series of interaction cells or cavities disposed adjacent each other sequentially along the axis of the tube. The electron stream passes through each cell along the axis of the tube and electromagnetic coupling is provided between the cavity and the electron stream. Each cavity is also coupled to an adjacent cavity by means of a coupling hole in the end Wall defining the cavity. Generally, in the past, these coupling holes between adjacent cells have been alternately disposed on opposite sides of the axis. When coupling holes are so arranged, the folded waveguide type .of energy propagation may be visualized as involving energy traversing the length of the tube which enters each cavity from one side, crosses the stream and leaves the cavity from the other side, thus traveling a sinuous or tortuous, extended path.

In practical traveling-wave tubes the electron stream is projected along the axis of the tube through minimum sized holes in the interaction cells or cavities or, more generally, as proximate as possible to the slow-wave structure for maximum interaction therewith. Accordingly, the stream must be focused and constrained to such a path by focusing means to prevent excessive impingement of electrons on the slow-wave circuit. This is generally done by immersing the electron stream in a strong axial magnetic field which tends to provide the required constraint so that the electron stream may pass as closely as possible to the slow-wave structure without excessive interception of the electrons by the slow-wave structure. Generally, in the past, such an axial constraining magnetic field has been provided by aligning the traveling-wave tube concentrically Within a long solenoid Wound of a conductor carrying relatively large electrical currents. As traveling-wave tubes have been designed and developed for higher and higher power handling capabilities, such solenoids have had to be larger and larger and carry more current, thus requiring not only very large auxiliary power supplies but also special cooling means. Further, if the focusing solenoid in such arrangement fails, even momentarily, to supply the focusing magnetic field, the electron stream may immediately destroy the slow-wave structure or other parts of the traveling-wave tube due to the large current in the electron stream. Obviously, the weight and bulk of such a traveling-wave tube, including its solenoid focusing system, make such a tube impractical for many mobile operations.

Another common scheme for focusing a traveling-wave tube involves the use of large permanent magnets with a pole piece at each end of the traveling-wave tube to provide a strong axial magnetic field. Here too, the size and weight of the magnet are prohibitive for many airborne or other mobile applications.

There has relatively recently been developed a system for periodically focusing traveling-wave tubes. In accordance with this system, generally, there is provided external to the electron stream and to the slow-wave structure and to the vacuum envelope therefor, a series of axially short permanent magnets disposed about the periphery of the envelope sequentially along the length of the tube. Placed between adjacent ones of these magnets is a pole piece which extends radially inwardly as nearly as possible to the envelope. The axial gap between adjacent pole pieces is used to provide a magnetic lens, the focusing effects of which extend into the slow-wave structure for constraining the electron stream. Again, as higher and higher power is desired in the traveling-wave tube such a scheme becomes less effective in providing adequate focusing because insufiicient magnetic material can be placed between adjacent pole pieces or because the pole pieces themselves saturate and provide inadequate lens fields. A formidable wall against further development has been reached when the pole pieces have been placed to provide gaps as closely as possible to the slow-wave structure and the maximum amount of magnetic material has been provided between the pole pieces. In other words the best conventional magnetic system which produces the field for the lenses cannot be brought close enough to the electron stream, thus limiting the magnetic field produced on the axis to a value inadequate for focussing high power electron streams.

One of the many advantages of employing a travelingwave type of amplifier is that there is no theoretical limit to the magnitude of gain or amplification which may be obtained in a single tube with, therefore, a minimum number of fallible components, for example, a single electron gun and power supply. Further, additional gain may be had without additional power; one need only increase the length of the interaction structure until the desired gain is achieved. However, this advantage of traveling-wave tubes is generally, not realizable to the fullest extent because when the gain of the tube exceeds the total of the losses and signals are reflected from the high signal level end to the low signal level end, regenerative oscillations occur to destroy the usefullness of the tube as an amplifier.

In the past this problem has been solved to some extent by preventing reflections from reaching the input end of the tube. Generally, this solution is to dispose attenuating, lossy material at a point along the slow-wave structure to absorb the reflections. Such an attenuator generally also absorbs the forward traveling or desired signal, however, the modulated electron stream effectively unidirectionally couples a major portion of the desired signal through the attenautor.

However, many difficult problems arise when it is sought to employ such a type of isolating attenuator in a folded waveguide or interconnected cell type of slow-wave circuit. For example, microwave attenuators or terminations are generally tapered so that the index of refraction seen by the propagating signals is gradually changed from that of air or vacuum to an absorbing medium. With a helix the lossy material may be placed externally to the conductor and simply tapered as desired; however, in a folded waveguide circuit the signal permeates the volume of the waveguide and more or less independently excities each cell as a separate circuit. Thus an attenuator may not be smeared or tapered from cell to cell to provide the necessary impedance match.

In addition there must be provided sufficient attenuating substance of the correct composition at just the correct position to absorb all the energy without deleterious reflection and without destroying the periodicity of the circuit or the focussing means. Further, the absorbing material must be kept adequately cool to preclude its decomposition due to the very large amounts of energy which it, to be effective, must dissipate.

It is accordingly an object of the present invention to provide an improved structure for providing high gain in a traveling-wave tube, without danger of producing harmful oscillations.

Another object of this invention is to provide means for severing interaction circuits into a plurality of nonoscillating electron stream coupled amplifying sections.

A further object of this invention is to provide an extremely simple structure for effectively severing the interaction circuit of a traveling-wave tube without causing undesired reflections or mismatch or without introducing excessive heating.

Yet another object of this invention is to provide an economical means, easily constructed and installed, for severing the slow wave structure of a traveling-wave tube into sections.

A further object of this invention is to provide eflicient nonreflecting coupling between isolating cells and adjacent interaction cells along the length of a slow wave structure.

These and other objects of the present invention are achieved by a structure which utilizes a slow wave structure comprising a plurality of interaction cells serially arranged along the path of an electron beam. Certain of the cells are specially constructed to provide the desired isolation of groups of the cells from each other. The traveling-wave tube is thus severed into a plurality of amplifying sections, each one of which is substantially isolated from the others except by electron stream coupling. The isolation means between adjacent amplifier sections comprises a cell similar in overall respects to an interaction cell but which is partially filled with resistive attenuating material and which further includes an actual short circuit vane or septum to substantially completely isolate adjacent sections except by the unidirectionally traveling electron stream. In accordance with specific features of this invention, the attenuating means are so arranged as to minimize mismatches and heating while achieving the desired degree of attenuation.

Further novel features of this invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the following description, taken in conjunction with the accompanying drawings which are not drawn to scale and in which hke reference numerals refer to like parts, and in which:

Fig. 1 is an overall view partly in longitudinal section and partly broken away of a traveling-wave tube constructed in accordance with the present invention;

Fig. 2 is a detailed longitudinal sectional view of a portion of the tube illustrated in Fig. 1',

Fig. 3 is an exploded view of a set of typical elements included in the structure of an embodiment of the present invention; and

Fig. 4 is a detailed exploded view of a typical isolator section of the traveling-wave tube of Fig. 1.

Referring to the drawings and their description, at number of features are shown for purposes of completeness of discussion of a traveling-wave tube according to the present invention, which features are not claimed in the present application but are claimed and described more fully in applications assigned to the assignee of the present application and filed concurrently herewith: Self Aligning Traveling-Wave Tube and Method, by T. Leonard and E. I. Flannery, Serial Number 764,886, new Patent No. 2,957,102, dated October 18, 1960; Periodically Focussed Traveling-Wave Tube, by D. I. Bates, H. R. Johnson, and O. T. Purl, Serial Number 764,884; and Periodically Focussed Traveling-Wave Tube With Tapered Phase Velocity, by D. I. Bates, Serial Number 764,885, now Patent No. 2,956,200, dated October 11, 1960.

Referring with more particularity to Fig. 1, there is shown a traveling-wave tube 12 utilizing a plurality of annular disc-shaped focusing magnets 14. In the example of this figure, these are permanent magnets and are diametrically split, as shown in later figures, to permit their being easily slipped between assembled adjacent ones of a series of ferro-magnetic pole pieces 16, which are also shown in more detail in the later figures. The system of pole pieces 16 and magnets 14 form both a slowwave structure and envelope 18.

Coupled to the right hand or input end of the slowwave structure 18 is an input waveguide transducer 20 which includes an impedance step transformer 22. A flange 24 is provided for coupling the assembled travelingwave tube 12 to an external waveguide or other microwave transmission line (not shown). The construction of the flange 24 includes a microwave window (not shown) transparent to radio frequency energy but capable of maintaining a pressure differential for maintaining a vacuum within the traveling-wave tube 12. At the output end of the tube 12. shown in the drawing as the left-hand end, an output transducer 26 is provided which is substantially similar to the input impedance transducer 20.

An electron gun 28 is disposed at the right hand end, as shown in the drawing. of the traveling-wave tube 12 and comprises a cathode 30 which is heated by a filament 32. The cathode 30 has a small central opening 3-4 to aid in the axial alignment of the gun assembly with the remainder of the traveling-wave tube 12. The cathode 30 is secured about its periphery by a cylindrical shielding member 36 which is constructed in a manner to fold cylindrically, symmetrically back upon itself to form a double cylindrical shield and an extended thermal path from the cathode 30 to its outer supporting means. Such support and an electrical, highly conductive path to the cathode is thus achieved while providing considerable thermal insulation for the cathode and filament due to the extended or tortuous path for heat conduction, as well as because of the multiple cylindrical shielding against radiant heat which is provided by the cylinders shown. For additional details of this type of gun construction, see the patent to I A. Dallons, No. 2,817,039, entitled Cathode Support, issued December 17, 1957, and assigned to the assignee of the present invention.

A focusing electrode 38 supports the cylindrical shielding member 36. The focusing electrode 38 is generally maintained at the same potential as that of the cathode 30 and is shaped to focus the electron stream emitted by the cathode .in a well-collimated, high perveance beam ,of electrons which traverses the slow-wave structure 18 and electromagnetically interacts with microwave energy being propagated therealong. The electron gun configuration is in accordance generally with the teachings in the Patent No. 2,817,033, by G. R. Brewer, which issued December 17, 1957, entitled Electron Gun, which is assigned to the assignee of the present invention, and to which reference may be made for a more detailed explanation. The focusing electrode 38 is in turn supported by a hollow cylindrical support 40 which extends from the periphery of the focusing electrode to the right hand end of the traveling-wave tube 12. Its opening is hermetically sealed with a metal to ceramic seal 42 by means of a sealing flange 14 made of a material having a low coefficient of thermal expansion, such as Kovar. The right hand extremity of the cylindrical support 40 is supported by an annular flange member 46, which also may be of Kovar, and which is sealed in turn to a hollow ceramic supporting tube 48. The ceramic tube 48 further thermally insulates the inner intensively heated members of the electron gun 28 and also provides electrical insulation between the cathode-beam focusing assembly and the higher potential accelerating anode 52. Substantially encasing the electron gun 28 and secured to the central or radio frequency structure of the traveling-wave tube 12 is a hollow cylinder 59, which may be Kovar, to which is sealed the ceramic cylinder 48, thus completing the vacuum envelope about the right hand end of the traveling-wave tube 12.

At the left hand end of the tube 12, as viewed in Fig. 1, there is shown a cooled collector electrode 60 which has a conically-shaped inner surface 62 for collecting the elec trons from the high power electron stream and dissipating their kinetic energy over a large surface. The collector electrode is supported within the end of a water jacket cylinder 64 which is in turn supported by an end plate 66. A water chamber 68 is thus formed between the outer surface of the collector electrode 62 and the inner cylindrical surface of water jacket 64. A water input tube 70 supplies cool water to this chamber and a water output tube 72 exhausts the heated water out of the water chamber 68. Thus, considerable power may be dissipated without destruction of the collector electrode. Although water has been specified, obviously, other liquids or gases may be used as coolants.

The end plate 66 is sealed to a supporting cylinder 74, which may be of Kovar, and which is in turn sealed to a ceramic insulating cylinder 76. This ceramic insulating cylinder 76 is sealed at its opposite end to another Kovar supporting cylinder 73, which is in turn supported and sealed to the slow-wave structure end disc 80. The collector 62, the end plate 66, the supporting cylinders 74 and 73 and the ceramic insulating cylinder 76 are all coaxially supported in alignment with the axis of the traveling-wave tube 12.

For vacuum pumping or out-gassing the traveling-wave tube 12, a double-ended pumping tube 86 is connected to both of the input and output waveguide transducers 20 and 26. Outgassing during bake-out of the entire traveling-wave tube 12 is thus achieved as rapid-1y as possible. After the outgassing procedure, the tube 86 is separated from the vacuum pumping system by pinching off the tube at the tip 88.

The traveling-wave tube of the present invention may be severed into a number of amplifying sections 90, 92, 94, 96 and 5 15. Each of the amplifying segments or sections is isolated from the others by an isolator or termination section 1111), 162, 1.114 or 106. The structure of these isolating sections will be discussed in detail in connection with Figs. 2 and 4. It sufiices at this point to describe their function generally as providing a substantially complete radio frequency isolation between adjacent sections of the slow-wave structure 18 while at the same time allowing the electron stream to pass .tube 12.

is modulated at the output of each amplifying section.

The stream thus modulated, as .it enters :the subsequent amplifying .section, launches a new wave therein which is further amplified by the interaction between the new traveling wave and the electron stream. Thus there is provided unidirectional coupling through the electron stream between adjacent amplifying sections.

Referring with more particularity to Fig. 2, there is shown a detailed sectional view of a portion of the traveling-wave tube of Fig. 1. The ferromagnetic pole .pieces 16 are shown to extend radially inwardly to approximately the perimeter of .the .axial electron stream. Disposed contiguously about the electron stream in each case is a short drift tube 114 The drift tube 114 is in the form of a cylindrical extension or lip protruding axially along the stream from the surface of the pole piece 16.

Adjacent ones of the drift tubes are separated by a gap 112 which functions as a magnetic gap to provide a focusing lens for the electron stream and also as an electromagnetic interaction gap to provide interaction between the electron stream and microwave energy traversing the slow-wave structure.

At a radial distance outwardly from the drift tubes 110 each of the pole pieces 16 has a second short cylindrical extension 114 protruding from its surface. The extension 114 provides an annular shoulder concentric about the axis of the tube for aligning the assembly of the component elements of the slow-wave structure 18. Disposed radially within the extension 114 is a .conductive, non-magnetic circuit spacer 116 which has th form of an annular ring having an outer diameter substantially equal to the inner diameter of the cylindrical extension 114. The axial length of the spacer 116 determines the axial length of the microwave cavities 118 which are interconnected along the length of the slowwave structure 18. It is thus seen that the slow-wave structure may be assembled and self-aligned by stacking alternately the pole pieces 16 and the spacers 116. Each spacer 116 has two annular channels 121) in which, during the stacking procedure, a sealing material, such as a brazing alloy, is placed. When the slow-Wave structure 18 is assembled, it may be placed in an oven within a protective non-oxidizing atmosphere and heated so that the brazing alloy in the channels melts and fuses or .brazes the adjacent members of the slow-wave structure 18 together to form a vacuum tight envelope. The spacers 116 are fabricated of a nonmagnetic material, such as copper, thus providing a highly conductive cavity wall, while not magnetically shorting out the focusing gaps 112. The entire interior surfaces of the cavities are preferably plated with a highly conductive material such as a thin silver or gold plating 121.

For interconnecting adjacent interaction cells, a coupling hole 122 is provided in each of the ferromagnetic pole pieces 16, the more detailed shape and orientation of which will be described in connection with the description of Fig. 3 below. Also disposed between adjacent pole pieces 16 are the focusing magnets 14 which are annular in shape and fit angularly or azimuthally symmetrically about the cylindrical shoulder extensions 114. The magnets 14 may be diametrically split to facilitate their being applied to the slow-wave structure 18 after it has been otherwise assembled. The axial length of the magnets 14 is substantially equal to the axial spacing between adjacent pole pieces 16, and their radial extent is approximately equal to or may be, as shown, greater than that of the pole pieces 16. To provide the focusing lenses in the gaps 112, adjacent ones of the magnets 14 are stacked with opposite polarity, thus causing a reversal of the magnetic field at each successive lens along the tube.

Referring to a typical isolator section 100, there is shown a substantial continuity of the pole piece-magnetspacer assembly. However, the pole pieces 124 at either end of the isolator section and the spacer 126 are somewhat modified, with respect to pole piece 16 and spacer 116 respectively, which will be shown with greater clarity in Fig. 4. It is sufficient here to point out that attenuating material, which may be in the form of lossy ceramic buttons 128 which extend from within a coupling hole 122 through the special spacer 126 and partially into the wall of the pole piece 124 opposite the coupling hole. The spacer 126 forms a pair of modified cavities 130 which lie opposite respective ones of the coupling holes 122 and which are substantially filled with the lossy attenuating material.

The two cavities 130 are substantially isolated from each other by a short circuiting vane, shown in a later figure, and are isolated from interaction with the electron stream by means of a central portion of the special spacer which has the form of a ring having substantially the same radial dimensions as the drift tubes 110 and which extends between two of the drift tubes 110 as shown, in a manner to substantially shield the electron stream from the slow-wave structure in the region of the isolator section 100.

Along the length of the slow-wave structure 18, individual ones of the pole pieces 16 are spaced by axial distances as represented by a, b, c, and d. In a preferred arrangement of the traveling-wave tube of the present invention, these distances and the associated length of the spacers 116 may be slightly varied with respect to each other so that the effective axial length of the interaction cavities is successively increased toward the output or collector end. This is done in order to decrease the axial phase velocity of the traveling waves so that the desired interaction between the electron stream and the traveling waves will continue to a maximum degree even though the electrons are slowed down toward the collector end.

Referring to Fig. 3, one set of the plurality of pole pieces, magnets and spacers is shown for purposes of describing more clearly how the individual elements of the slow-wave structure 18 are fabricated and assembled. A typical pole piece 16 is shown twice in the figure, once in plan and once in side elevation. A typical magnet 14 and a typical spacer 116 are shown in side elevation only.

Referring to the side elevation view of the pole piece 16, the orientation of the pole piece 16 concentrically about the electron stream is shown. Substantially immediately surrounding the electron stream is the short drift tube 110 which extends axially in both directions normal to the plane of the pole piece 16. The remainder of the pole piece extends radially outwardly from the drift tube 110 as shown. Positioned radially in between these two extremes are the cylindrical shoulder extensions 114 which extend axially outwardly from both faces of the pole piece 16.

The outer diameter of the cylindrical extension 114 supports the focusing magnet 14 coaxially about the electron stream, while the inner diameter of the extension 114 rests against the outer periphery of the spacer 116. The inner diameter of the spacer 116 determines the outer dimension of the interaction cell which is formed between adjacent ones of the pole pieces 16. Before assembly, a sealing material is placed in the channels 120,

which are continuous annular grooves in the end surfaces of the spacers 116.

An off-center coupling hole 122 is provided through each of the pole pieces 16 to provide the transfer of radio frequency energy from cell to cell along the slowwave structure 18.

The size, shape and orientation of the coupling hole 122 may be more clearly seen in the plan view thereof at the left hand end of Fig. 3. The drift tube is shown as having an inner radius r slightly larger than the radius of the electron stream and having an outer radius r which substantially defines the inner radius of the interaction cell. The kidney-shaped coupling hole 122 may be formed by an end mill having a diameter ex tending from r; to 1' The end mill is pressed through the thickness of the pole piece 16 centered upon the arc of a circle 132. The end mill, or preferably the work, may then be swung along this are keeping its center on the circle 132. The work is rotated through an arc of angle a where a may be any angle between zero degrees and, for example, approximately 60. Thus, the kidney-shaped coupling hole 122 lies between a radius 1' and r, and has circular ends of diameter r4r Disposed radially outwardly from the coupling hole 122 is a cylindrical shoulder extension 114, the inner radius of which is designated r and is substantially equal to the outer radius of the spacer 116. The inner radius r of the spacer 116 determines the outer dimension of the radio frequency interaction cell. The outer radius of the extension 114, designated as r,, is substantially equal to the inner radius of the magnet 14. The outer radius of the pole piece 16 is designated r and the outer radius of the magnet 14 is designated r For angular alignment purposes during assembly, one or more sets of holes 134 are provided through the pole pieces 16 to hold them in a predetermined angular position with respect to each other. A reference notch 136 may be provided on the periphery of each of the pole pieces 16 in order that one may always know from an observation of the outer surface of the assembled tube what the angular orientation of each pole piece is. In the example described here, the notch is always provided opposite the center of the kidney-shaped coupling hole 122.

Referring to Fig. 4, there is shown an exploded view of a typical one of the isolator sections shown in dotted lines in Fig. 1, for example, the isolator section 100. The isolator pole pieces 124 are shown in perspective to point out the manner in which they are modified from the typical circuit pole pieces 16. A pair of overlapping circular recessions 136 are provided in the face of each of the isolator pole pieces 124 toward the middle of the isolator section 100. The circular recessions 136 extend approximately half-way through the pole piece 124 and retain the enlarged head portions 138 of the attenuator buttons 128. The attenuator buttons 128 may be formed of a porous ceramic impregnated with carbon. This may be done by soaking the ceramic in a carbohydrate solution, such as sugar, and then baking the soaked piece in an oxygen-free atmosphere to leave a residue of carbon distributed uniformly throughout the volume of the ceramic.

The focusing magnet 14 is typical of the remainder of the focusing magnets and need not be specially modified for the isolator section. The special isolator spacer 126 fits radially within the cylindrical shoulder extensions 114 and has a pair of cavities 130 one each associated with a coupling hole 122. A web end portion closes the end of each of the cavities 130 except for a pair of overlapped openings 142 which are oriented respectively concentric with the circular recessions 136, but have a lesser diameter. The attenuator buttons 128 extend then from the depth of the recessions 136 through the openings 142 in the web end portion 140 through a cavity 13a to approximately half-way through the opposite coupling hole 122.

A circular shoulder 146 is provided on each side of the spacer 126 to receive the end of the drift tube 110 from each of the pole pieces. It is thus seen that the two cavities 130 are isolated from each other by a conductive mid-portion or vane 15%. The microwave energy in the slow-wave structure 18 to the left in the drawing of the isolator spacer 126 may enter coupling hole 122 of the left hand isolator pole piece shown in Fig. 4 and will intercept the ends of two of the attenuator buttons 128 approximately half-way through the coupling hole 122. Whatever fraction of the microwave energy is not absorbed and dissipated in that portion of the lossy ceramic may pass on into the associated cavity 130 where it will eventually be completely absorbed.

In exactly the same manner, microwave energy in the slow-wave structure to the right of the isolator section and traveling toward the isolator section will be substantially completely absorbed by the other termination.

In the operation of the traveling-wave tube 12, microwave energy traverses from right to left along the slow- Wave structure, being amplified first in section 98 due to its interaction with the electron stream. Near the output of this amplifying section, the traveling wave has grown and has caused considerable density modulation in the electron stream. At the first isolator section, section 106 in the drawing, the radio frequency energy in the slow-wave structure 18 is substantially completely absorbed. However, the modulated electron stream passes on into the next amplifier section, section 6, where it launches a new traveling wave in that section. The new traveling wave grows and is amplified by the electron stream until reaching its output end at the isolator section 104. The electron stream is further modulated and the RF energy in the slow-wave structure is again completely absorbed. This procedure is repeated until the highly modulated electron stream enters the output amplifier section 90 through the isolator section 109 and launches a high energy traveling wave upon the output section 90 of the slow-wave structure 18. The output of this final section is fed into the output waveguide through the transducer 26.

The isolator sections 100, 102, 104 and 106 each represents a loss of a few decibels of amplification. However, overall they vastly increase the amount of power amplification or gain which may be achieved in a single traveling-wave tube. The isolation sections isolate adjacent amplifying sections, thereby to preclude instability and oscillations due to reflections and to too great an amplification in a single traveling-Wave tube section.

It has been pointed out that the attenuating material inserted into the isolated cavities 130 (see particularly Figs. 2 and 4) does not fill the entire volume thereof but leaves some free space about the rods or buttons 128. This has been found empirically to be analogous to a tapered termination in a conventional section of waveguide. In other words, it is found that a combination of vacuum and lossy ceramic provides an excellent impedance match. Another factor in providing the best impedance match is the extent to which the attenuating material protrudes through the coupling holes 122.. It has been found that placing the ends of the buttons approximately midway through the coupling hole provides maximum impedance matching and places the discontinuity of dielectric at a non-critical point. As stressed above, one of the cavities 130 terminates one end of the amplifying section 90 and the other cavity 130 in the same isolator section 1011 terminates the other amplifying section 92. The two cavities 130 are then mutually isolated by the conductive septum or vane 150. Thus amplifying sections 90 and 92 are likewise isolated except by the unidirectional coupling of the electron stream.

Another serious problem solved here is the dissipation of heat from the lossy ceramic without damaging the composition thereof. In the present invention the ceramic buttons 128 make good thermal contact with portions of the coupling holes 122 and the enlarged head portions 138 have most of their surface imbedded against the recessions 136 and the web portion 140 and are therefore in excellent thermal contact with an appropriate one of the isolator pole pieces 124 and the isolator spacer 126.

Thus sufficient attenuator material is provided and at the correct point to absorb substantially all of the electromagnetic energy of the traveling waves while yet having little enough to provide an excellent impedance match. In addition the cylindrical buttons are selected to have a porosity factor of about 30% and are easy to form and impregnate with a lossy substance. The small axial opening 144 in the ceramic buttons aids in impregnating them and in addition speeds up the vacuum pumping of the finished assembly.

It is apparent that modification and changes may be made in the isolator sections without departing from the scope of the present invention; for example, the attenuating material may be secured or bonded as by brazing or gluing to the appropriate one of the isolator pole pieces instead of, or in addition to, retaining them by the web portion 140 and the openings 142 therein.

There has thus been disclosed a novel traveling wave tube which integrally combines a radio frequency slow wave structure with its own periodic focusing means, and which in particular includes means for axially severing the tube so that without destroying the periodic structure or the periodic focusing, the tube is successfully and advantageously divided into a plurality of stable, non-oscillating amplifying sections which may readily amplify millivolts into high power kilovolts. Such a unitary tube may thus replace as many as five or more conventional traveling wave or other types of tubes. Many advantages reside in achieving the functions of several tubes in one. For example, the smallest and lightest possible overall package is obtained; only single elements and auxiliary components need be supplied such as the electron gun and power supply and modulator; the fewer the number of tubes and components, the lower the probability of failure of the incorporating system; and as mentioned above, as much gain as is desired may be built into the tube at no expense of power.

We claim:

1. An electromagnetic structure for providing interaction between a stream of charged particles projected along a predetermined path and a radio frequency electromagnetic wave comprising: a combined electrically conductive magnetic means extending adjacently about said stream for providing a plurality of mutually radio frequency isolated groups of series of electromagnetically intercoupled interaction cells arranged along and in an electromagnetic interacting relationship with said stream of charged particles and including a series of magnetic lenses along said stream for focussing and constraining it to fiow along said path, each of said cells forming an interaction volume exposed to said stream and including in its end walls magnetic material for both conducting said electromagnetic wave and for providing said lens; and electromagnetic wave isolating means between adjacent ones of said groups comprising at least one of said cells having a substantial fraction of its internal volume filled with radio frequency attenuating material whereby said groups are intercoupled only by said stream of charged particles.

2. An electromagnetic structure for providing interaction between an electron stream projected along a predetermined path and radio frequency electromagnetic energy comprising in combination: electrically conductive magnetic means for providing a series of electromagnetically intercoupled interaction cells arranged along and in electromagnetic interaction relation with said electron stream for providing a series of magnetic lenses along and immediately contiguously about said stream for focussing and constraining it to flow along said path, each of said interaction cells including annular magnetic disc elements spaced along said path about said electron stream and extending radially from said electron stream to a predetermined outer radius, a hollow, cylindrical, nonmagnetic spacer element hermetically sealed between adjacent ones of said disc elements and disposed concentrically about said stream, the inner diameter of said spacer element determining the outer diameter of an associated interaction cell, and said interaction cells also including coupling means associated with ones of said disc elements for coupling said electromagnetic wave between adjacent ones of said interaction cells and disposed radially between said electron stream and the inner diameter of said spacer element, and attenuating material disposed within separated ones of said cells radially between said spacer element and said electron stream for severing said series of cells into a plurality of radio frequency isolated, electron stream coupled groups of cells.

3. A high power traveling-wave tube amplifier comprising: electron gun means providing an electron beam along a path coincident with the axis of said tube; an axially severed slow-wave structure including a plurality of segments in axial sequential alignment; radio frequency absorptive termination means disposed between each of said segments for attenuating radio frequency energy traversing the length of said slow-wave structure at the junctions between said segments, each of said segments comprising a plurality of radio frequency interaction cells electromagnetically intercoupled and coupled to said electron stream and being axially determined by a pair of conductive web portions extending radially outwardly from the periphery of said electron stream, each of said interaction cells being radially determined by a conductive, nonmagnetic cylindrical spacer secured along its ends to respective ones of said conductive web portions for axially spacing said web portions, said conductive web portions being of a paramagnetic, conductive material and extending radially outwardly beyond said conductive spacer to form a series of magnetic pole pieces extending radially outwardly from said conductive spacer and radially inwardly substantially to the periphery of said electron stream; a plurality of annular magnets having an axial length substantially equal to the axial spacing of said ferromagnetic web portions, having an inner diameter approximately equal to the outer diameter of said cylindrical spacer, and having an outer diameter greater than or approximately equal to that of said web portions, each of said magnetic web portion pole pieces being radially terminated about said electron stream with a short, hollow cylindrical drift tube extending axially away from said web portion, the juncture between said segments of said slow-wave structure comprising an interaction cell similar to one of said plurality of interaction cells but having within said cell sufiicient attenuating material to provide radio frequency isolation between adjacent ones of said segments whereby said ferromagnetic web portions provide periodic focussing for said electron stream exceedingly close to said electron stream while at the same time providing the end walls for said radio frequency interaction cells.

4. A high power, periodically focussed traveling-wave tube comprising: focussing magnet pole pieces which extend radially inwardly to approximately the perimeter of the electron stream and form the end walls of the intercoupled, interaction cells, the tube being severed into a plurality of stable, nonoseillating amplifying sections; radiofrequency isolator means disposed between adjacent pairs of said amplifying sections comprising: a pair of ferromagnetic pole pieces extending radially inwardly to approximately the perimeter of the axial electron stream and being axially separated by a distance substantially equal to the axial length of said interaction cells; an annular, nonmagnetic cylindrical spacer element extending and disposed concentrically between said pole pieces and being relieved along its axis to permit passage of said electron stream therethrough and having an outer diameter somewhat greater than that of said interaction cells, each of said pole pieces having a radio frequency coupling hole extending therethrough radially separated from the axial passage for said electron stream, said spacer element being relieved to form a pair of cavities mutually radio frequency isolated, one of said cavities being disposed adjacent one of said coupling holes in one of said pole pieces, and the other of said cavities being disposed adjacent the other of said coupling holes in the other pole piece whereby radio frequency energy may be coupled into said cavities from different ones of said amplifying sections while not propagating between said pair of cavities; and termination means at least partially within each of said cavities for terminating each of said different ones of said amplifying sections in a substantially resistive reflectionless termination.

5. In a periodically focussed traveling-wave tube in which the pole pieces for the focussing magnets extend radially inwardly to approximately the perimeter of the electron stream and form the end walls of the intercoupled, interaction cells and which is severed into a plurality of stable, nonoseillating amplifying sections, radio frequency isolator means disposed between adjacent pairs of said amplifying sections comprising: a pair of ferromagnetic pole pieces extending radially inwardly to approximately the perimeter of the axial electron stream and being axially separated by a distance substantially equal to the axial length of said interaction cells, an annular, nonmagnetic cylindrical spacer element extending and disposed concentrically between said pole pieces and being relieved along its axis to permit passage of said electron stream therethrough and having an outer diameter somewhat greater than that of said interaction cells, each of said pole pieces having a radio frequency coupling hole extending therethrough radially separated from the axial passage for said electron stream, said spacer element being relieved to form a pair of symmetrically disposed cavities mutually radio frequency isolated, one of said cavities being disposed adjacent one of said coupling holes in one of said pole pieces, and the other of said cavities being disposed adjacent the other of said coupling holes in the other pole piece whereby radio frequency energy may be coupled into said cavities from different ones of said amplifying sections while not propagating between said pair of cavities; and termination means at least partially within each of said cavities for terminating each of said different ones of said amplifying sections in a substantially resistive reflectionless termination.

6. In a high power, periodically focussed travelingwave tube wherein the slow wave structure includes a series of annular ferromagnetic pole piece discs aligned and axially spaced to form space periodic interaction cells along the electron stream and each having an integral axially extending ferromagnetic drift tube disposed contiguously thereabout, adjacent ones of said pole piece discs and drift tubes providing magnetic lenses in the region of said electron stream for focussing said stream, means for severing said tube into a plurality of electron stream coupled amplifying sections comprising a conductive nonmagnetic isolator spacer disposed in axial alignment between a pair of said pole piece discs, said pair of discs each having a coupling hole therein radially separated from said drift tube and diametrically opposed to each other for coupling radio frequency energy from adjacent ones of said amplifying sect-ions toward said isolator spacer; said isolator spacer being relieved to form a pair of diametrically opposed terminating cavities mutually isolated by a septum portion of said isolator spacer and each being coupled through the coupling hole of respective ones of said pair of pole pieces to one of said amplifying sections; attenuating termination means disposed in each of said cavities including an absorbing body of porous ceramic impregnated with a lossy substance partially filling each of said cavities, being in good thermal contact with said conductive isolator spacer and protruding partially into said coupling hole.

7. In a high gain, high power periodically focussed traveling-wave tube amplifier in which a series of substantially spaced periodic interaction cells and focussing lenses are aligned adjacent and sequentially along the axis of the tube, the end walls of said cells being also a portion of the magnetic circuit for said focussing lenses, isolating means for terminating said series of cells into electron stream coupled nonoscillating amplifying sections each comprising a group of said cells, said isolating means comprising: a modified pair of adjacent ones of said end walls each having a magnetic drift tube disposed contiguously about said coupling electron stream and extending axially from each of said end walls toward the other being however axially spaced to leave a magnetic focusing gap between the two opposing drift tubes, said modified end walls having each a coupling hole radially separated from said dn'ft tube and being disposed diametrically opposite to each other; a conductive and nonmagnetic isolator spacer disposed between said pair of end walls concentric about said stream, said spacer being hermetically bonded between said pair of end walls and being relieved to permit the passage of said stream and to form a pair of termination cavities diametrically opposed and mutually isolated by a conductive septum which also precludes interaction between said electron stream and electromagnetic energy in said cavities Without, however, appreciably aifecting the periodicity of said magnetic circuit or of said series of interaction cells, each of said cavities being coupled through one of said coupling holes to a different one of said amplifying sections; at least one cylindrical porous, ceramic attenuating body impregnated with carbon and disposed with its axis parallel to that of said stream partially within each of said cavities, said attenuating body extending the axial length of said cavity and partially through said coupling hole, said attenuating body also extending partially through the opposite one of said pair of end walls by means of a circular recess, of diameter equal to that of said cylindrical attenuating body, in said opposite end wall; means for retaining said attenuating body in good thermal contact with the surfaces of said circular recess and -for maintaining the axial position of said body with respect to the coupling hole partially into which said body extends.

References fitted in the file of this patent UNITED STATES PATENTS 2,636,948 Pierce Apr. 28, 1953 2,637,001 Pierce Apr. 28, 1953 2,741,718 Wang Apr. 10, 1956 2,810,854 Cutler Oct. 22, 1957 2,843,775 Yasuda July 15, 1958 2,847,607 Pierce Aug. 12, 1958 ,939,993 Zublin et a1. June 7, 1960 

